This invention relates to signal processors for eliminating certain blind speeds associated with radar moving target indication, also known as clutter cancellation, by the use of dual displaced phase center antenna (DPCA) processing. When radars are used for surveillance, it is desirable to reduce the workload imposed on the radar operator. For this purpose, it has long been the custom to provide moving target indication (MTI) to eliminate those radar returns from the display which represent non-moving or stationary targets, and to display only those returns which represent moving objects. In this way, only targets of importance are displayed. Such targets may include aircraft or missiles. In the case of a simple stationary surveillance radar, cancellation of stationary-target returns is readily accomplished by subtraction or combining with mutually inverse amplitude or phase the returns resulting from two successive transmitted pulses. Since the returns rom stationary targets remain the same from pulse to pulse, the subtraction cancels or eliminates those signals. Only those targets which have moved during the inter-pulse interval do not cancel completely, and result in a display.
Another method for detecting the presence of moving targets is to compare the returned signal from the target with a signal which is in frequency and phase coincidence with the transmitted pulse. This may be accomplished, for example, by applying the returned signal to a fast Fourier transformer for determining the frequency spectrum of the returned signal. The frequency spectrum of the signal returned from a moving target will include a Doppler frequency shift or component attributable to the motion of the target, whereas the returns from stationary targets will be at the transmitter frequency. Thus, a signal appearing at the output of a filter having a frequency other than the frequency of the transmitted pulse indicates the presence of a moving target, and in addition the magnitude of the frequency offset provides an indication of the radial velocity of the target.
When the surveillance radar system is mounted on a moving platform, the motion of the platform creates a Doppler shift of returns from stationary targets. FIG. 1 illustrates in perspective or isometric view an airplane 10 travelling in a +X direction, directing antenna beams 12 and 16 in two directions, either simultaneously or sequentially. A first beam 12 is directed in forward direction X and down toward the terrain. Even if the cross-section of the antenna beam is circular, its impingement on the terrain below can be represented as a roughly oval or elliptical region 14. If the axial cross-section of antenna beam 12 is elliptical, region 14 will be even more elongated. Second beam 16 is directed downward and in a lateral direction relative to direction of motion +X, and intercepts the terrain in a similar elongated region 18.
The aircraft motion creates Doppler frequency components which accompany the signal reflected or returned from each region 14 and 18. The Doppler components arise from the components of the relative velocity between the aircraft and the illuminated terrain which are directed along the aircraft-to-terrain axis, i.e., a line joining the aircraft and the particular point or the terrain causing the reflection. For this reason, the greatest Doppler frequency components arise from leading edge 20 of region 14. The Doppler components of the returned signal decrease in frequency as they approach trailing edge 22 of region 14. Because of the relatively large distance or angular separation between leading and lagging edges 20 and 22 of region 14, and for fast moving platform, the frequency spread of the Doppler components is relatively large. When beam 16 is directed exactly broadside to the direction of motion X, there is a center axis 28 of region 14 from which the returned signals have zero Doppler frequency component. The Doppler frequency spread for the lateral beam 16, which is due to the distance between leading point 24 and lagging point 26 of region 18 is the same as for the forward beam 12. The spread of frequencies in all directions depends upon the forward velocity of the vehicle and also upon the beamwidth of the antenna. Much of the terrain which is illuminated by the beam will result in a broad spread of frequencies which will compete with the Doppler frequencies of the targets of interest.
A system of transmitting frequency-jumped pulses can be used to cancel clutter in a radar system mounted on a moving vehicle, by compensating for the frequency spread in the forward direction and for relatively small angles on each side of the forward direction. For the side looking portions of the radar, the frequency jump system is less effective at clutter elimination.
In order to provide compensation for the Doppler spread in any direction from a moving vehicle, the signal processing system known as displaced phase center antenna (DPCA) was introduced. In DPCA, two antennas are used whose phase centers are separated in the direction of motion of the vehicle. The two antennas are coupled together for transmission to form a single transmitting antenna having a phase center centered between the phase centers of the individual antennas. After each pulse is transmitted, the two antennas are mutually separated for the receive mode of operation.
The returns from the target received by the two antennas in DPCA processing are individually processed by multiplying each of the returns by a weighting function for reducing the frequency spread attributable to the pulse nature of the transmission. The weighted signals representing returns from the target are each applied to a Fourier transform processor, in that the frequency components are sorted or divided into frequency components within predetermined frequency bands. A delay equal to the inter-pulse interval (the time between transmitted pulses) is introduced into one of the channels of received signals. Within each frequency band, the difference is taken between the signals in the two channels. This arrangement subtracts the signal received at one antenna from the signal received at the other antenna, with a delay. In effect, the returns resulting from two separate, sequential pulse transmissions are subtracted, with the lagging antenna occupying the location of the leading antenna for reception of the "delayed" return. For stationary terrain, the two signals which are subtracted are substantially identical whereby their subtraction results in cancellation. This compensates for or eliminates the vehicle motion on the returns from stationary targets. Returns from stationary targets are therefore suppressed. Moving targets appear as residual difference signals in one or more of the frequency bands. The presence of a moving target is identified by applying the difference signal in each frequency band to an individual threshold circuit which responds to a signal exceeding a threshold value. The particular frequency band within which a return is found establishes the radial velocity of the moving target.
FIG. 2 illustrates as a solid-line plot 30 the amplitude response in dB versus target range rate or radial velocity for a particular DPCA system. In FIG. 1, 0 dB represents maximum response. It can be seen that for target radial velocities or range rates of zero, the system has very low response. In addition, at a target range rate of approximately 400 knots there is a substantial null or decrease in the amplitude. Such nulls are known as "blind speeds". Although not illustrated in FIG. 2, similar decreases in amplitude occur periodically at velocities related to the pulse repetition interval: EQU V.sub.B =.lambda./(2 PRI) (1)
where .lambda. is the transmitted wavelength and PRI is the transmitted pulse recurrence interval.
It is clearly undesirable for a surveillance radar to have to have reduction in amplitude response for certain targets. In order to move the null illustrated in FIG. 1 to a higher velocity away from the target velocities of interest, it appears from equation (1) to only be necessary to decrease the pulse repetition interval (PRI) of the system.
The PRI in a DPCA system is selected to cause the phase center of the lagging antenna to move into the position of the leading antenna at the vehicle velocity. Two antennas 50 and 60 are located as illustrated in FIG. 3, moving to the left with vehicle velocity V.sub.s. Each antenna has a length L in the direction of motion. Antenna 50 has a phase center 52 centrally located thereon, and antenna 60 likewise has a phase center 62. The distances from phase centers 52 and 62 to the leading and lagging edges of their respective antennas are equal to L/2. Lagging phase center 62 must move a distance (L/2) in order to assume the position previously occupied by the phase center of the transmit antenna, which lies midway between phase centers 52 and 62. In order for this to occur during one inter-pulse period PRI, EQU PRI=L/(2V.sub.s) (2)
where L is the distance between phase centers. Consequently, lowering the PRI in order to raise the velocity at which the amplitude null of FIG. 2 occurs (the blind speed) requires decreasing the distance between phase centers, or increasing the vehicle velocity. For a given vehicle velocity, one way to decrease the distance between phase centers is by decreasing L which decreases the size of the antenna. However, this decreases its gain, which is undesirable. Alternatively, the antenna could be made as an array in which sections could be rendered inoperative so as to provide a full aperture L when desired but a reduced aperture when a blind speed increase was desired. But this is similar to reducing the aperture which is undesirable because of the reduction in gain. If it should be desired to increase the PRI without changing the total length of the antenna, the spacing between phase centers would have to increase by overlapping the apertures. Overlapping apertures requires complex feed and beamformer structures, and may be limited due to size and weight constraints. All of the above solutions are therefore undesirable, because of size and weight limitations associated with vehicles such as aircraft.
Improved processing is desired for reducing or eliminating the effects of amplitude nulls or blind speeds in the response of radar system without the bulk and complexity associated with the systems necessary to change the effective size or configuration of the antenna.